1. Field of the Invention
This invention is related to transmitting ceramic materials and, more particularly, to ternary phosphides useful as infrared materials in the 8 to 12 micron wavelength range in such applications as satellite windows and missile domes. This invention also relates to a new electrochemical synthesis of ternary phosphides.
2. Description of the Prior Art
Phosphides, sulfides and compounds with mixed sulfide-phosphide anions are used as 8-12 nanometer transmitting materials because they have excellent thermal, mechanical and optical properties. These materials are used for sensor windows and domes on satellites, missiles and other similar devices. However, high speed missiles operating at microwave and radar frequencies suffer from boresight errors and error slopes due to changes in the radome material dielectric properties caused by aerothermal heating. Currently available or developing materials such as ZnS, ZnSe, GaAs, Ge and CaLa.sub.2 S.sub.4 do not meet the proposed optical requirements or stringent environmental operating standards.
In general, materials with strong chemical bonds exhibit good thermochemical properties but poor IR transmission. Conversely, most materials showing good IR transmission possess weaker chemical bonds and poor thermal and mechanical properties. For example, cubic crystalline structures often exhibit useful IR transmission, but these same structures have lower strengths than non-cubic analogues and are more susceptible to thermal shock. Compounds with the chalcopyrite structure (ABC.sub.2), such as ZnSiP.sub.2 and ZnGeP.sub.2, have desirable optical and mechanical properties for use as IR transmitting ceramics in the 8-12 micron wavelength region. Furthermore, MgGeP.sub.2 has been reported to go through a high temperature phase transition from the chalcopyrite to the diamond structure. Thus a method exists for toughening the chalcopyrite phase.
The preferred method of synthesizing ternary phosphides and mixed sulfur-phosphorous compounds is by direct combination of the elements typically using chemical vapor transport with a halogen, flux growth with fluxes such as Sn, Sb, Pb and Zn, or by crystal growth under pressure. Electrochemical synthesis in molten metaphosphates is a known method of preparing simple transition metal binary phosphides. However, there have been few reported electrochemical syntheses of non-transition metal phosphides and the applicants have found no reports of the electrochemical synthesis of ternary or higher phosphides. The electrochemistries of group IV components of the ternary compounds are difficult to control. Silicon and germanium are semiconductors that passivate readily in oxidizing media, particularly at the elevated temperatures needed to form the ternary compounds. No such passivation of the group IV components occurs in non-oxidizing media such as molten chlorides and fluorides. However, melts showing electrochemistry favorable for the group IV components generally have an unfavorable phosphorus chemistry. Conversely, melts which show favorable phosphorus chemistry are generally oxide rich and passivate and precipitate the group IV species as an insoluble oxide.
The production of silicon containing ternary phosphides presents a particularly difficult challenge since the choice of a silicon source is a critical factor in determining the products that are produced and the yield of those products. The design of a practical silicon source is a significant problem since the use of a pure silicon cathode is not practical because of its high electrochemical resistance. Similarly, silicon has a high melting temperature and is nearly insoluble in the molten tin or molten zinc used in the cathode. The choice of a practical silicon source is further limited by the fact that no zinc silicides are known.